t4034 (diff --word-diff): style suggestions

Rearrange code to be easier to browse:

 - first data
 - then functions
 - then test assertions

Mark up inline test vectors as

  cat >vector <<-\EOF
	data
	data
  EOF

for visual scannability.  Use words like "set up" for tests that set
up for other tests, to make it obvious which tests are safe to skip.
Use repeated function calls instead of a loop for the
language-specific tests, so the invocations can be easily tweaked
individually (for example if one starts to fail).

This means if you add a new subdirectory to t4034/, it will not be
automatically used.  I think that's worth it for the added
explicitness.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>